The invention relates to an apparatus for reproducing information from an optically readable record carrier on which information is stored in the form of a track of optically detectable areas which alternate with intermediate areas. Such an apparatus comprises an optical system for projecting a light beam on an optical detector via the record carrier. The detector comprises four sub-detectors situated in separate quadrants of an imaginary X-Y-coordinate system whose origin is situated on the optical axis of the optical system, with the X-axis extending effectively in the track direction and the Y-axis extending effectively transversely of the track direction. The apparatus further includes a first adder circuit for adding the signals supplied by diagonally disposed first and third sub-detectors to form a first signal, a second adder circuit for adding the signals supplied by at least the other two sub-detectors to form a second signal, and a tracking-error signal generator for deriving a tracking-error signal from the first and the second signals.
Such an apparatus is described in Netherlands Patent Application No. 7600842 corresponding to U.S. Pat. No. 4,057,833 and may be employed in, inter alia compact-disc digital audio players.
In the known apparatus a tracking-error signal is obtained by shifting the phase of the second signal through 90.degree., multiplying said phase-shifted signal by the first signal, and subsequently passing it through a low-pass filter. Such a method of generating the tracking-error signal is found to be unsatisfactory, in particular in compact-disc players because the 90.degree. phase shift cannot be obtained due to the varying instantaneous frequency of the signal being read and because the generated tracking-error signal is influenced considerably by the recorded information signal.